In a typical wireless communication network, wireless terminals, also known as mobile stations and/or user equipments (UEs), communicate via a Radio Access Network (RAN) to one or more core networks. The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” or “eNodeB”. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not collocated. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole wireless communication network is also broadcasted in the cell. One base station may have one or more cells. A cell may be a downlink (DL) and/or uplink (UL) cell. A DL cell being a cell that a wireless terminal is connected to primarily for communications in the DL, and an UL cell being a cell that a wireless terminal is connected to primarily for communications in the UL. The base stations communicate over the air interface operating on radio frequencies with the wireless terminals within range of the base stations.
A Universal Mobile Telecommunications System (UMTS) is a third generation wireless communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) and/or High Speed Packet Access (HSPA) for wireless terminals. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some versions of the RAN as e.g. in UMTS, several base stations may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System (EPS) have been completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the base stations are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the base stations, e.g. eNodeBs in LTE, and the core network. As such, the Radio Access Network (RAN) of an EPS has an essentially “flat” architecture comprising base stations without reporting to RNCs.
Dual connectivity is a feature defined from the wireless terminal perspective wherein the wireless terminal may simultaneously receive from and transmit to at least two different network points, such as base stations. Dual connectivity is one of the features that are considered for standardization under the small cell enhancements study item within 3GPP Rel-12.
Dual connectivity is defined for the case when the aggregated network points operate on the same frequency or on separate or different frequencies. It is further foreseen that from the wireless terminal perspective, the wireless terminal may apply some form of Time Division Multiplexing (TDM) scheme between the different network points that the wireless terminal is aggregating in some scenarios. This implies that the communication on the physical layer to and from the different aggregated network points may not be truly simultaneous.
Dual connectivity as a feature bears many similarities with carrier aggregation and CoMP; the main differentiating factor is that dual connectivity is designed considering a relaxed backhaul and less stringent requirements on synchronization requirements between the network points. This is in contrast to carrier aggregation and CoMP wherein tight synchronization and a low-delay backhaul are assumed between connected network points. FIG. 1 shows a relationship between base stations of two different power levels in a wireless communication network.
TDD (Time Division Duplex) Systems
TDD systems have a feature that allows for asymmetric UL/DL allocations and thus, a possibility to adjust the used time-frequency resources in terms of instantaneous traffic. The UL/DL allocations may correspond to one of the seven different UL/DL configurations that are defined for LTE-TDD systems as shown in FIG. 2. This is contrary to Frequency Division Duplex (FDD) systems, where a bandwidth is either allocated to DL or UL operations regardless of the traffic pattern and the need at a certain node.
However, the cost of UL/DL dynamic resource adaptation is the cross interference between UL and DL that arises when neighboring cells use different TDD configurations, causing interference between base station to base stations or wireless terminal to wireless terminals, which does not occur in FDD systems. In some cases, these interferences can become very severe and impact the performance of the wireless communication network in a detrimental way.